Re: [Chicken-users] Controlling output of processes

[Lasse Kliemann]

* Message by -Peter Bex- from Fri 2010-03-12:
> On Fri, Mar 12, 2010 at 07:30:34PM +0100, Lasse Kliemann wrote:
> > Hello Peter, thank you for the answer. I tried it out, but it 
> > does not work like the thing I wanted. Here is an example:
> > 
> > (receive
> >    (p-stdout p-stdin p-pid p-stderr)
> >    (process* "ls" '("-l"))
> >    (with-input-from-port p-stdout (lambda () (process-wait p-pid))))
> > 
> > When I run this, I receive no output. The goal was that my 
> > program hands the output of "ls -l" through. So this program was 
> > supposed to give exactly the output of "ls -l". You know what I 
> > mean?
> 
> Hi Lasse,
> 
> I think I do.  The idea of calling with-input-from-port is that you
> put the code that reads from that port inside the thunk.  Just waiting
> for the process isn't enough; there has to be some code that actually
> reads out the data from stdout.
> 
> Here's an example:
> (receive (p-stdout p-stdin p-pid p-stderr)
>     (process* "ls" '("-l")) 
>   (let ((output (with-input-from-port p-stdout read-lines)))
>     (process-wait p-pid)
>     output))
> 
> This returns a list of strings, with one entry for each line that is
> output by "ls".  If you would like something else, you can change
> read-lines by  (lambda () <whatever you want to do>)

I'd rather hand it through directly, but this can be done with a 
loop involving read-line and write-line. What I can't have this 
way is standard output and standard error intermixed in the order 
as output comes - but for this I can use process-run.

So if I want both, I use process-run. If I want one of them 
(either standard output or standard error) I use the above with a 
read-line and write-line loop. If I want none of them, I use the 
above and simply ignore the ports.

Instead of handing output through with a loop, it may be a 
cleaner approach to implement a more flexible version of 
process*. I've seen that the implementation is in Scheme, in 
posixunix.scm starting from line 2336. So it should be possible 
without too much hassle. I'll consider this.

Kind regards,
Lasse

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